Applications are known where metal-oxide oxygen carriers are utilized for the delivery of oxygen via reduction of the oxygen carrier. One such application which has been investigated extensively is chemical looping combustion. Chemical looping combustion systems generally utilize a fuel reactor, an air reactor, and a metal oxide oxygen carrier undergoing reduction in the fuel reactor and oxidation in the air reactor. The reduction in the fuel reactor is facilitated by close contact between a fuel and the oxygen carrier. The subsequent oxidation of the carrier in the air reactor is an exothermic process and a stream of N2 is exhausted from the air reactor and carries the heat of oxidation to an attached power generation island.
Chemical looping combustion cycles provide potentially significant advantages, enhanced reversibility of the two redox reactions offers improved efficiencies over traditional single stage combustions, where the release of a fuel's energy occurs in a highly irreversible manner. Further, with appropriate oxygen carriers, both redox reactions can occur at relatively low temperatures, allowing a power station to more closely approach an ideal work output without exposing components to excessive working temperatures. Additionally, and significantly, chemical looping combustion can serve as an effective carbon capture technique. Of the two flue gas streams generated, one is comprised of atmospheric N2 and residual O2, but sensibly free of CO2, while the second stream is comprised of CO2 and H2O, and contains almost all of the CO2 generated by the system. It is relatively uncomplicated to remove the water vapor, leading to a stream of almost pure CO2. For these reasons, chemical looping combustion systems have been extensively investigated.
It is understood that high reactivity and oxygen utilization of the oxygen carrier is desired in chemical looping combustion systems in order to limit the solid inventories utilized in the various processes. Generally, the amount of the bed material in each reactor and the solid circulation rates between reactors mainly depends on the oxygen carrying capacity of the carriers. As a result, an important characteristic of a successful oxygen carrier is its reactivity in both reduction and oxidation cycles. To increase, reactivity, oxygen carrier particles are often prepared by depositing a metal oxide phase on an inert support such as SiO2, TiO2, ZrO2, Al2O3, YSZ, bentonite, and others, in order to stabilize the metal loading and increase exposed surface area over repeated reduction-oxidation cycles.
Magnesium oxide (MgO) has additionally been utilized to foster the support of certain metal oxides such as Fe2O. This has generally been conducted by sintering a Fe2O3/MgO mixture at temperatures exceeding about 1000° C. and stabilizing the oxide by generating MgFe2O4. See e.g., Jin et al., “Development of a Novel Chemical-Looping Combustion: Synthesis of a Solid Looping Material of NiO/NiAl2O4.” Ind. Eng. Chem. Res. 38 (1999). Similarly magnesium has been utilized as a component in various supporting spinels. See e.g., Ryden et al. “Fe2O3 on Ce-, Ca- or Mg-Stabilized ZrO2 as Oxygen Carrier for Chemical-Looping Combustion Using NiO as Additive,” AIChE Journal, Vol. 56, No. 8 (2010), and see Johansson et al., “Investigation of Fe2O3 with MgAl2O4 for Chemical-Looping Combustion,” Ind. Eng. Chem. Res. 43 (2004), and see Adanez et al., “Progress in Chemical-Looping Combustion and Reforming technologies,” Prog. Energ. Combust. 38 (2012). Generally speaking, in these oxygen carriers, MgO has been absent as a discrete component. In cases where MgO has been identified within the oxygen carrier, reduction temperature have been limited to a maximum of 700° C., and very slow or no reaction has been reported. See Jin et al., and see Adanez et al. MgO has additionally been utilized as a discrete component in conjunction with Fe2O3 and MnO2 in order to enhance fluidization, however the resulting mixture exhibits CO selectivity on the order of 90%. See U.S. Pat. No. 2,607,699 to Corner et al., issued Aug. 19, 1952.
Disclosed here is an oxygen carrier comprised of a metal oxide and MgO promoter particles for the chemical looping combustion of a gaseous hydrocarbon at temperatures greater than about 725° C. The oxygen carrier maintains MgO as a discrete component over numerous redox cycles, and demonstrably improves the percentage combustion and oxygen utilization of the metal oxide. The MgO component does not function as a support material for increasing the surface area of the oxygen carrier. Additionally, the oxygen carrier generates CO2 and H2O combustion gases with a substantial absence of CO and H2. The effect is temperature dependent and is generally not observed at temperatures below about 700° C. In a particular embodiment, the metal oxide is Fe2O3, and MgO comprises between 5 weight % (wt. %) and 25 wt. % of the Fe2O3/MgO mixture.
The objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.